Voltage fluctuations on high voltage AC transmission lines can be reduced by installing static reactive power generators (sometimes known as VAR generators) along the transmission lines. Voltage regulation is based on the fact that in an essentially inductive transmission line, voltage increases if capacitive current is injected into the line by, for example, connection of a shunt capacitor across the line/load. Alternatively, voltage can be decreased by connecting an inductor across the line (or removing a previously connected capacitor). Static VAR generators may be switched across the line using electromagnetic relay devices controlled by a predetermined timer or using a thyristor (or other semiconductor) switching control network such as that disclosed in U.S. Pat. No. 4,353,024 to Gyugyi.
A problem facing many utilities is controlling shunt compensation on voltage buses especially where voltage is already regulated by load tap change (LTC) transformers. In an LTC transformer, the low side line voltage delivered to the load is monitored and regulated by a conventional, fine tuning LTC controller. Such a controller measures actual low side voltage, compares it with the desired value, and then adjusts the position where the load tap makes contact with the low side LTC transformer coil, e.g., via a control signal to a motorized tap changer. Typical LTC transformers may have 16 or 32 tap positions, with each position being representative of some fractional portion of rated voltage. Thus, for example, a one position tap change on a 32 tap LTC transformer would cause a relatively small line voltage change as compared with the rated or desired output voltage.
In operation, an LTC transformer compares the secondary voltage with both a minimum and a maximum voltage threshold. If either threshold is exceeded, a timer is started. If the time exceeds a predetermined delay period, the LTC controller moves the tap to increase or decrease the secondary voltage as necessary.
LTC transformers function well to effect small changes in voltage. However, large voltage fluctuations require switching of shunt reactances to ensure that sufficient reactive power is provided to the system, end-users and customers such that secondary voltage can be held essentially constant. Since the LTC controller is already monitoring and regulating the secondary distribution voltage, a shunt reactance control unit cannot also directly control that secondary distribution voltage. As a result, most utilities typically follow a fairly rigid load cycle to estimate roughly when a reactance element, such as a capacitor bank, should be switched in shunt across the load to offset a decrease in the secondary distribution voltage from an increased load. After capacitor bank switching, the LTC controller gradually adjusts the tap to return the low side voltage to the desired value. This rigid scheduling is far from optimal because it fails to accurately respond to actual system needs (as opposed to scheduled estimates) and to detect abnormal system conditions.
The present invention seeks to overcome these problems by flexibly coordinating the LTC fine tune controller and shunt reactance switching. More specifically, the present invention provides voltage and power regulation using a programmable logic controller for controlling shunt capacitor switching in order to attain the following exemplary objectives:
(1) maintain distribution and transmission voltages;
(2) track station loading;
(3) complement the action of LTC transformers;
(4) provide sufficient deadband and time delays to avoid hunting; and
(5) detect and compensate for abnormal system conditions.
In one embodiment, the present invention provides a system for coordinating shunt reactance switching in a power distribution substation which includes a transformer having a primary voltage and a secondary voltage for supplying low voltage power to a load. Voltage and power meters are provided for measuring the primary voltage and the reactive (or real) power flowing to the load. A programmable logic controller receives as one set of inputs measurements of primary voltage and reactive (or real) power flowing to the load, and as another set of inputs, predetermined ranges establishing high and low limits for the primary voltage and the reactive (or real) power. Based on these inputs, the programmable logic controller connects or disconnects at least one shunt reactance across the load to maintain the load voltage substantially constant.
The present invention is particularly well suited to coordinate shunt reactance switching with LTC transformers. An LTC controller monitors the secondary voltage and adjusts the tap contact position in response to secondary voltage variations. The present invention is also applicable as well to power transmission lines for delivering secondary voltages to a variety of loads. In both environments, a primary voltage of the LTC transformer or the transmission line system is monitored along with reactive power to a load.
The programmable logic controller uses the predetermined ranges for primary voltage and reactive power to determine a deadband range of operation in which no reactance switching is necessary. The deadband range and switching determination are based upon a mathematical model formulated as a function of (1) primary or high side voltage and (2) power flowing to the load. If that function exceeds a calculated deadband range, an error is calculated, and integrated, over time. When the integrated error exceeds a preset value, reactance switching occurs.
The present invention also includes a method for coordinating shunt reactance switching with an LTC transformer having a high voltage side and a low voltage side for supplying low voltage to a load including the steps of: (1) adjusting the position of an adjustable tap contacting windings on the low voltage side in response to variations in the low side voltage; (2) measuring the high side voltage and reactive power flowing to the load; and (3) switching at least one reactance in shunt with the load to maintain a substantially constant low side voltage based on the measurements in step (2) and on predetermined ranges for the high side voltage and reactive power. The method may further include calculating a deadband range of operation in which no reactances are switched and outside of which reactances are switched.
The present invention will allow for controlled switching at a location within the "load" network that is physically outside of the power distribution substation. That is, the switched reactances can be at other buses which are electrically connected through low side system impedances (lines, cables, etc.) to the LTC transformer. This switching of remotely located reactive elements will require communication circuitry of the variety readily available through normal SCADA (system control and data acquisition) systems.
The present invention also includes a method for regulating shunt reactance-switching in a power transmission system providing a plurality of distribution voltages including a first voltage and a second voltage for supplying power to a load including the steps of (1) measuring the first voltage and reactive power flowing towards the load, and (2) switching at least one shunt reactance to maintain the second voltage substantially constant based on the measurements made in step (1) and predetermined ranges for the first voltage and reactive power. The method further includes the steps of inputting predetermined ranges which include a minimum second voltage, a maximum second voltage, a minimum reactive power, and a maximum reactive power, and calculating a deadband range of operation in which no reactances are switched and outside of which reactances are switched based on those predetermined ranges.